Write an outline of the main points you'll address in your paper.
Research all of your main points, and also research some general information to include in your introduction.
Figure out a compelling reason why others will want to read your paper and write a sentence that clearly states that reason.
Include your sentence about why others will find your topic compelling as the last sentence to your introductory paragraph.
Combine several sentences that discuss generalities about fish with several sentences about the main points you will cover in your paper to make the two sentence groups flow together and form your introductory paragraph. Use a phrase like "In addition," However," or "Interestingly," to smoothly combine the sentences.